2.1 Microtubules
The cytoskeleton of eukaryotic cells consists of an extensive network of microfilaments, microtubules and intermediate filaments. Microtubules play an important role in mitosis. α-, β-, and γ-tubulin subunits are eukaryotic cytoskeleton proteins that are responsible for the formation of microtubules. Microtubules are hollow cylinders which are comprised of α,β-tubulin heterodimers, joined end-to-end along the microtubule axis. γ-tubulin is involved in microtubule organization. Once formed, the microtubules exist in an equilibrium, with tubulin dimers constantly being added to one end of the microtubule and removed from the opposite end. This equilibrium allows for control of the length of the microtubule and such control is essential for the microtubules to carry out their numerous functions in cells.
During cell division microtubules are responsible for transporting the set of daughter chromosomes to each individual daughter cell. In particular, during prophase, the DNA in the nucleus is replicated and the two sets of genetic material are organized into the individual sets of daughter chromosomes. Toward the end of prophase, microtubules grow from the centrosomes at either end of the dividing parent cell and toward the two identical sets of chromosomes. This growing bundle of microtubules forms a structure known as the mitotic spindle. During prometaphase, the microtubules attach themselves to the chromosomes, and upon entry into anaphase, the microtubules destabilize and shorten, drawing the daughter chromosomes apart to their respective daughter cells at opposite ends of the dividing cell. Thus, microtubules are intimately involved with the cell division process.
2.2 Cancer and Neoplastic Disease
Currently, cancer therapy involves surgery, chemotherapy and/or radiation treatment to eradicate neoplastic cells in a patient (see, for example, Stockdale, 1998, “Principles of Cancer Patient Management”, in Scientific American: Medicine, vol. 3, Rubenstein and Federman, eds., Chapter 12, Section IV). All of these approaches pose significant drawbacks for the patient. Surgery, for example, can be contraindicated due to the health of the patient or can be unacceptable to the patient. Additionally, surgery might not completely remove the neoplastic tissue. Radiation therapy is effective only when the irradiated neoplastic tissue exhibits a higher sensitivity to radiation than normal tissue, and radiation therapy often elicits serious side effects. (Id.)
With respect to chemotherapy, there are a variety of chemotherapeutic agents available for treatment of neoplastic disease. Specific examples of chemotherapeutic agents include drugs that target tubulin (e.g., inhibit tubulin polymerization or stability or tubulin stability) or microtubules such as colchicine (an alkaloid extracted from the meadow suffron), the vinca alkaloids (e.g., vincristine, vinblastine and vinorlbine) and the taxanes (e.g., paclitaxel (Taxol®) and docetaxel (Taxotere®). Colchicine exerts its cytotoxic effect by binding to the tubulin heterodimer at a single high-affinity binding site known as the colchicine site. This binding induces an alteration in the structure of the dimer and hinders the assembly of the dimers into microtubules. The colchicine binding site displays affinity for a diverse group of molecular structures, including, but not limited to, the podophyllotoxins, steganacin, the chalcones, nocodazole and TN-16. Exposure of rapidly dividing cells such as cancer cells to Colchicine causes the disappearance of the mitotic spindle and blocks the cells in M phase of the cell cyle and eventually kills the cells. The vinca alkaloids bind to a site on β-tubulin known as the vinca alkaloid binding site, resulting in a destabilization of the tubulin dimers. The poisoned dimers can then be incorporated into the microtubule polymer and prevent further growth of the microtubule. The taxanes bind directly to tubulin subunits of intact microtubules, stabilize the microtubules, and inhibit depolymerization or stability. When the dividing cell enters anaphase, the stabilized microtubules are prevented from contracting and are not able to draw each set of daughter chromosomes to their respective daughter cells. Thus, cell division cannot take place and the cells are blocked in M phase of the cell cycle and eventually apoptosis results.
Despite the availability of a variety of chemotherapeutic agents, traditional chemotherapy has many drawbacks (see, for example, Stockdale, 1998, “Principles Of Cancer Patient Management” in Scientific American Medicine, vol. 3, Rubenstein and Federman, eds., ch. 12, sect. 10). Almost all chemotherapeutic agents are toxic, and chemotherapy can cause significant, and often dangerous, side effects, including severe nausea, bone marrow depression, immunosuppression, etc. Additionally, many tumor cells are resistant or develop resistance to chemotherapeutic agents through multi-drug resistance. Therefore, there is a significant need in the art for novel compounds, compositions, and methods that are useful for treating cancer or neoplastic disease with minimal or no side effects. Further, there is a need for cancer treatments that provide cancer-cell-specific therapies with increased specificity and decreased toxicity.
2.3 Inflammatory Disorders
Inflammation plays a fundamental role in host defenses and the progression of immune-mediated diseases. The inflammatory response is initiated in response to injury (e.g., trauma, ischemia, and foreign particles) and infection (e.g., bacterial or viral infection) by a complex cascade of events, including chemical mediators (e.g., cytokines and prostaglandins) and inflammatory cells (e.g., leukocytes). The inflammatory response is characterized by increased blood flow, increased capillary permeability, and the influx of phagocytic cells. These events result in swelling, redness, warmth (altered heat patterns), and pus formation at the site of injury or infection.
Cytokines and prostaglandins control the inflammatory response, and are released in an ordered and self-limiting cascade into the blood or affected tissues. This release of cytokines and prostaglandins increases the blood flow to the area of injury or infection, and may result in redness and warmth. Some of these chemicals cause a leak of fluid into the tissues, resulting in swelling. This protective process may stimulate nerves and cause pain. These changes, when occurring for a limited period in the relevant area, work to the benefit of the body.
A delicate well-balanced interplay between the humoral and cellular immune elements in the inflammatory response enables the elimination of harmful agents and the initiation of the repair of damaged tissue. When this delicately balanced interplay is disrupted, the inflammatory response may result in considerable damage to normal tissue and may be more harmful than the original insult that initiated the reaction. In these cases of uncontrolled inflammatory responses, clinical intervention is needed to prevent tissue damage and organ dysfunction. Diseases such as rheumatoid arthritis, osteoarthritis, Crohn's disease, asthma, allergies or inflammatory bowel disease, are characterized by chronic inflammation.
Current treatments for inflammatory disorders involve symptomatic medications and immunosuppressive agents to control symptoms. For example, nonsteroidal anti-inflammatory drugs (NSAIDs) such as aspirin, ibuprofen, fenoprofen, naproxen, tolmetin, sulindac, meclofenamate sodium, piroxicam, flurbiprofen, diclofenac, oxaprozin, nabumetone, etodolac, and ketoprofen have analgesic and anti-inflammatory effects. However, NSAIDs are believed not to be capable of altering progression of the disease. (Tierney et al. (eds), Current Medical Diagnosis & Treatment, 37 ed., Appleton & Lange (1998), p 793). Moreover, NSAIDs frequently cause gastrointestinal side effects, affect the lower intestinal tract causing perforation or aggravating inflammatory bowel disease, produce renal toxicity and prolong bleeding time. Corticosteroids are another class of drugs that are commonly used to control inflammatory symptoms. Corticosteroids, like NSAIDs, do not alter the natural progression of the disease, and thus, clinical manifestations of active disease commonly reappear when the drug is discontinued. The serious problem of untoward reactions resulting from prolonged corticosteroid therapy (e.g., osteoporosis, increased risk of infection, increased appetite, hypertension, edema, peptic ulcers, psychoses) greatly limits its long-term use.
Low doses of immunosuppressive agents such as cytotoxic agents are also commonly used to in treatment of inflammatory disorders. For example, methotrexate, an antagonist of folic acid, is often used in treatment of psoriasis, rheumatoid arthritis and other inflammatory diseases. Methotrexate, like other cytotoxic agents, frequently causes stomatitis, erythema, slopecia, nausea, vomiting, diarrhea, and damage to major organs such kidney and liver. The long-term usage of immunosuppressive agents usually leaves the patient defenseless to infections.
New treatments for inflammatory disorders are constantly being sought. In particular, any new treatment that reduces the dosage and/or frequency of administration of agents currently being used, or is capable of making a currently used treatment more effective is constantly being sought.
2.4 Central Nervous System Disorders
Central nervous system disorders affect a wide range of the population with differing severity. Generally, one major feature of this class of disorders includes the significant impairment of cognition or memory that represents a marked deterioration from a previous level of functioning. Dementia, for example, is characterized by several cognitive impairments including significant memory deficit and can stand alone or be an underlying characteristic feature of a variety of diseases, including Alzheimer disease, Parkinson disease, Huntington disease, and Multiple Sclerosis to name but a few. Other central nervous system disorders include delerium, or disturbances in consciousness that occur over a short period of time, and amnestic disorder, or discreet memory impairments that occur in the absence of other central nervous system impairments.